My name is***** and I am sorry to hear that Kingsley is having dry heaving.
A food allergy is definitely my first choice of possibilities when I see a dog with chronic intestinal signs, but is otherwise feeling good. Science Diet is a chicken and corn based food, but the mobility also has a significant portion of fish protein and oils in it.
What were you feeding before? I would read the ingredient labels and see if there is anything different between the two foods.
Also, glucosamine treats for dogs are often beef flavored, so that could be it as well.
I would consider putting him back on his old food and see if he returns to normal. If so, then try the food alone or the glucosamine alone to see if one of those is the trigger for the intestinal signs.
If he can't tolerate the Science Diet product, Royal Canin and Purina also have mobility diets as well.
Does this help? What other questions do you have?